irqchip/gic-v3-its: Add VLPI map/unmap operations

In order to let a VLPI being injected into a guest, the VLPI must
be mapped using the VMAPTI command. When moved to a different vcpu,
it must be moved with the VMOVI command.

These commands are issued via the irq_set_vcpu_affinity method,
making sure we unmap the corresponding host LPI first.

The reverse is also done when the VLPI is unmapped from the guest.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>